1. Field of the Invention
The present invention is directed to methods and apparatus for detecting incidents or behaviors in a medical image scan of a subject, in certain embodiments to detecting movements of the subject.
2. Description of the Prior Art
In the medical imaging field, several imaging schemes are known. For example PET (Positron Emission Tomography) is a method for imaging a subject in 3D using an injected radio-active substance which is processed in the body, typically resulting in an image indicating one or more biological functions.
Positron Emission Tomography (PET) is becoming an increasingly useful and widespread modality because of its ability to tailor tracers to study specific aspects of physiology and metabolism in vivo. Developments in scanner hardware, reconstruction algorithms and tracers continue to improve the quality of the images; but, as they do so, the effects of motion have become an increasingly important limitation on the overall quality of the information that can be obtained.
The first problem that motion causes is the obvious blurring of the reconstructed image. For ‘dynamic’ PET, this has implications beyond the loss of spatial detail as it can be hard to tell if the variation in activity (or shape) is due to the movement or from the physiological activity.
The second problem is due to the need to perform attenuation correction during reconstruction. Some of the photons that should be detected will instead be absorbed by other tissues. This process can be compensated for using an attenuation map, a model of the attenuation experienced along different projection directions through the body. This is usually estimated from a CT scan taken immediately before (or after) the PET acquisition on a hybrid scanner. However any motion between the attenuation and emission scans means that the attenuation map is no longer valid and the corrections applied during reconstruction will inevitably introduce artifacts, over- or under-correcting for the attenuation in some regions. This can influence the diagnostic conclusions reached.
The third problem is that iterative reconstruction is non linear, and therefore the image that results from reconstruction of data from a mix of positions is not necessarily a mix of images of the subject in those positions. This point is seemingly ignored in practice, but felt to be significant.
FIG. 1 is a demonstration of the non-linearity of reconstruction, using a basic 2D OSEM algorithm. From left to right: (100) image resulting from reconstruction of a data set comprising of two distinct positions; (102) image resulting from summation of two separately reconstructed images, using the data from each position separately; (104) image resulting from summation of half of two separately reconstructed images, using double the amount of the data from each position separately, such that each reconstruction uses the same amount of data as in the far left case. The bottom row shows histograms of the differences T2−T1 (106); T3−T1 (108) and T3−T2 (110) for points inside the body of the rat.
There are measurable differences in the pattern of noise between the third case and the other two, and the difference between the third and either of the others is significantly greater than between the first two. This indicates that the extra amount of data available to the reconstruction algorithm in the first case relative to the second is not having a significant effect (i.e. producing a smoother image), but the extra data does in the third—where the extra data is self consistent.
Given knowledge of when the subject moved the scan could be split into sections, each corresponding to a single position. There exist ways to register two images together and thus remove the effects of motion, but the detection of when the motion occurred remains a problem.
One example of a disturbance or disruptive even in an imaging scan is a ‘twitch’: an abrupt motion between otherwise comparatively stable positions, as opposed to a periodic motion like the beating heart (where the practice of gating is widely used).
Twitches have received little attention in the literature; most work on motion has either been centered around registration (where the division of the scan into segments is assumed to have been done somehow) or been in the context of gated motions like breathing or the beating of the heart.
For macro-motions, like twitches, most of the reported approaches involve external optical cameras tracking a target (points that are easily identified, for example infra-red reflector beads) attached to the subject: for example Picard, Y. & Thompson, C. “Motion correction of PET images using multiple acquisition frames” Medical Imaging, IEEE Transactions on, Medical Imaging, IEEE Transactions on, 1997, 16, 137-144. This sort of external monitoring is rarely done in practice; the time required to setup the tracking apparatus imposes too much delay in the clinical workflow and too much stress on patients.
Nehmeh, S. A.; Erdi, Y. E.; Rosenzweig, K. E.; Schoder, H.; Larson, S. M.; Squire, O. D. & Humm, J. L. “Reduction of Respiratory Motion Artifacts in PET Imaging of Lung Cancer by Respiratory Correlated Dynamic PET: Methodology and Comparison with Respiratory Gated PET” J Nucl Med, 2003, 44, 1644-1648 proposes a method that does not need external sensors, rather a radioactive point source attached to the subject is tracked from the PET scanner's data. This reduces, but does not eliminate, the additional workflow. Introducing an extra radiation source, would, however, mean that the additional setup would require proper procedures for handling radioactive materials.
A similar method is suggested in Lu, W. & Mackie, T. R. “Tomographic motion detection and correction directly in sinogram space” Physics in Medicine and Biology, 2002, 47, 1267, but the authors assume that there are sufficiently clear points of high activity within the subject to track and so do not propose the use of an additional, external, point source. This work seems to be only theoretical however.
In short, there is little done to detect twitches in the art: established methods are not seen as being practical for routine use because of the burden entailed.